In spite of these disappointments, Galileo was not deterred, on his return home, from continuing his independent researches into natural phenomena. The most important invention of those times, to which he was led by the works of Archimedes, too little regarded during the Middle Ages, was his hydrostatic scales, about the construction and use of which he wrote a treatise, called “La Bilancetta.” This, though afterwards circulated in manuscript copies among his followers and pupils, was not printed until after his death, in 1655.
Galileo now began to be everywhere spoken of in Italy. The discovery of the movement of the pendulum as a measurement of time, the importance of which was increasingly recognised, combined with his novel and intellectual treatment of physics, by which the phenomena of nature were submitted, as far as possible, to direct proof instead of to the a priori reasoning of the Aristotelians, excited much attention in all scientific circles. Distinguished men of learning, like Clavius at Rome, with whom he had become acquainted on his first visit there in 1587,[18] Michael Coignet at Antwerp, Riccoboni, the Marquis Guidubaldo del Monte, etc., entered into correspondence with him.[19] Intercourse with the latter, a distinguished mathematician, who took the warmest interest in Galileo’s fate, became of the utmost importance to him. It was not merely that to his encouragement he owed the origin of his excellent treatise on the doctrine of centres of gravity, which materially contributed to establish his fame, and even gained for him from Del Monte the name of an “Archimedes of his time,” but he first helped him to secure a settled and honourable position in life. By his opportune recommendation in 1589, the professorship of mathematics at the University of Pisa, just become vacant, was conferred on Galileo, with an income of sixty scudi.[20] It is indicative of the standing of the sciences in those days that, while the professor of medicine had a salary of two thousand scudi, the professor of mathematics had not quite thirty kreuzers[21] a day. Even for the sixteenth century it was very poor pay. Moreover, in accordance with the usage at the Italian Universities, he was only installed for three years; but in Galileo’s needy circumstances, even this little help was very desirable, and his office enabled him to earn a considerable additional income by giving private lessons.
During the time of his professorship at Pisa he made his grand researches into the laws of gravitation, now known under the name of “Galileo’s Laws,” and wrote as the result of them his great treatise “De Motu Gravium.” It then had but a limited circulation in copies, and did not appear in print until two hundred years after his death, in Albèri’s “Opere complete di Galileo Galilei.” Aristotle, nearly two thousand years before, had raised the statement to the rank of a proposition, that the rate at which a body falls depends on its weight. Up to Galileo’s time this doctrine had been generally accepted as true, on the mere word of the old hero of science, although individual physicists, like Varchi in 1544, and Benedetti in 1563, had disputed it, maintaining that bodies of similar density and different weight fall from the same height in an equal space of time. They sought to prove the correctness of this statement by the most acute reasoning, but the idea of experiment did not occur to any one. Galileo, well aware that the touchstone of experiment would discover the vulnerable spot in Aristotelian infallibility, climbed the leaning tower of Pisa, in order thence to prove by experiment, to the discomfiture of the Peripatetic school, the truth of the axiom that the velocity with which a body falls does not depend on its weight but on its density.[22]
It might have been thought that his opponents would strike sail after this decisive argument. Aristotle, the master, would certainly have yielded to it—but his disciples had attained no such humility. They followed the bold experiments of the young professor with eyes askance and miserable sophistries, and, being unable to meet him with his own weapons of scientific research, they eagerly sought an opportunity of showing the impious and dangerous innovator the door of the aula.
An unforeseen circumstance came all at once to their aid in these designs. An illegitimate son of the half-brother of the reigning Grand Duke,—the relationship was somewhat farfetched, but none the less ominous for Galileo—John de’ Medici, took an innocent pleasure in inventing machines, and considered himself a very skilful artificer. This ingenious semi-prince had constructed a monster machine for cleaning the harbour of Leghorn, and proposed that it should be brought into use. But Galileo, who had been commissioned to examine the marvel, declared it to be useless, and, unfortunately, experiment fully confirmed the verdict. Ominous head-shakings were seen among the suite of the deeply mortified inventor. They entered into alliance with the Peripatetic philosophers against their common enemy. There were cabals at court. Galileo, perceiving that his position at Pisa was untenable, voluntarily resigned his professorship before the three years had expired, and migrated for the second time home to Florence.[23]
His situation was now worse than before, for about this time, 2nd July, 1591, his father died after a short illness, leaving his family in very narrow circumstances. In this distress the Marquis del Monte again appeared as a friend in need. Thanks to his warm recommendation to the Senate of the Republic of Venice, in the autumn of 1592 the professorship of mathematics at the University of Padua, which had become vacant, was bestowed on Galileo for six years.[24] On 7th December, 1592, he entered on his office with a brilliant opening address, which won the greatest admiration, not only for its profound scientific knowledge, but for its entrancing eloquence.[25] His lectures soon acquired further fame, and the number of his admirers and the audience who eagerly listened to his, in many respects, novel demonstrations, daily increased.
During his residence at Padua, Galileo displayed an extraordinary and versatile activity. He constructed various machines for the service of the republic, and wrote a number of excellent treatises, intended chiefly for his pupils.[26] Among the larger works may be mentioned his writings on the laws of motion, on fortification, gnomonics (the making of sun-dials), mechanics, and on the celestial globe, which attained a wide circulation even in copies, and were some of them printed long afterwards—the one on fortification not until the present century;[27] others, including the one on gnomonics, are unfortunately lost. On the wide field of inventions two may be specially mentioned, one of which was not fully developed until much later. The first was his proportional circle, which, though it had no special importance as illustrative of any principle, had a wide circulation from its various practical uses. Ten years later, in 1606, Galileo published an excellent didactic work on this subject, dedicated to Cosmo de’ Medici, and in 1607 a polemical one against Balthasar Capra, of Milan, who, in a treatise published in 1607, which was nothing but a plagiarism of Galileo’s work disfigured by blunders, gave himself out as the inventor of the instrument. Galileo’s reply, in which he first exhibited the polemical dexterity afterwards so much dreaded, excited great attention even in lay circles from its masterly satire.[28] The other invention was a contrivance by which heat could be more exactly indicated. Over zealous biographers have therefore hastened to claim for their hero the invention of the thermometer, which, however, is not correct, as the instrument, which was not intended to measure the temperature, could not be logically called a thermometer, but a thermoscope, heat indicator. Undoubtedly it prepared the way by which improvers of the thermoscope arrived at the thermometer.[29]
Before proceeding further with Galileo’s researches and discoveries, so far as they fall within our province, it seems important to acquaint ourselves with his views about the Copernican system. From a letter of his to Mazzoni, of 30th May, 1597,[30] it is clear that he considered the opinions of Pythagoras and Copernicus on the position and motion of the earth to be far more correct than those of Aristotle and Ptolemy. In another letter of 4th August of the same year to Kepler, he thanks him for his work, which he had sent him, on the Mysteries of the Universe,[31] and writes as follows about the Copernican system:—
“I count myself happy, in the search after truth, to have so great an ally as yourself, and one who is so great a friend of the truth itself. It is really pitiful that there are so few who seek truth, and who do not pursue a perverse method of philosophising. But this is not the place to mourn over the miseries of our times, but to congratulate you on your splendid discoveries in confirmation of truth. I shall read your book to the end, sure of finding much that is excellent in it. I shall do so with the more pleasure, because I have been for many years an adherent of the Copernican system, and it explains to me the causes of many of the appearances of nature which are quite unintelligible on the commonly accepted hypothesis. I have collected many arguments for the purpose of refuting the latter; but I do not venture to bring them to the light of publicity, for fear of sharing the fate of our master, Copernicus, who, although he has earned immortal fame with some, yet with very many (so great is the number of fools) has become an object of ridicule and scorn. I should certainly venture to publish my speculations if there were more people like you. But this not being the case, I refrain from such an undertaking.”[32]
In an answer from Grätz, of 13th October of the same year, Kepler urgently begs him to publish his researches into the Copernican system, advising him to bring them out in Germany if he does not receive permission to do so in Italy.[33] In spite of this pressing request of his eminent friend, however, Galileo was not to be induced to bring his convictions to the light yet, a hesitation which may not appear very commendable. But if we consider the existing state of science, which condemned the Copernican system as an unheard of and fantastic hypothesis, and the religious incubus which weighed down all knowledge of nature irrespective of religious belief, and if, besides all this, we remember the entire revolution in the sphere both of religion and science involved in the reception of the Copernican system, we shall be more ready to admit that Galileo had good reason to be cautious. The Copernican cause could not be served by mere partisanship, but only by independent fresh researches to prove its correctness, indeed its irrefragability. Nothing but the fulfilment of these conditions formed a justification, either in a scientific or moral point of view, for taking part in overturning the previous views of the universe.